In recent years, a so-called noise-canceling headphone has entered the market in response to the growing needs of improvement in comfortability in an environment where there is too much noise, typically an aircraft cabin or the like. The noise-canceling headphone is a headphone apparatus using an active noise control technique in which a control sound in antiphase to a noise is actively outputted, whereby the noise is reduced (e.g., Patent Document 1).
Hereinafter, a conventional noise-canceling headphone will be described with reference to FIG. 20. FIG. 20 shows a configuration of the conventional noise-canceling headphone. Here, FIG. 20 shows a view seen from above a head of a user 90. In FIG. 20, the user 90 faces upward.
As shown in FIG. 20, the noise-canceling headphone comprises a headband 91, a left ear case 92a, a right ear case 92b, a left ear speaker 93a, a right ear speaker 93b, a left ear microphone 94a, a right ear microphone 94b, a left ear control section 95a and a right ear control section 95b. The left ear case 92a is placed near a left ear of the user 90. The right ear case 92b is placed near a right ear of the user 90. The left ear case 92a and the right ear case 92b are connected by the headband 91. The left ear speaker 93a is provided within the left ear case 92a. The right ear speaker 93b is provided within the right ear case 92b. The left ear microphone 94a is provided within the left ear case 92a. The right ear microphone 94b is provided within the right ear case 92b. 
Here, the left ear case 92a and the right ear case 92b have spaces formed therein, respectively. These spaces are acoustically independent from each other. Here, being acoustically independent means that an acoustic state is such that a gain of an electroacoustic transfer function between the spaces is sufficiently small.
The left ear microphone 94a detects a noise arriving in the left ear case 92a. The left ear microphone 94a outputs, as a detection signal eL to the left ear control section 95a, a noise signal based on the detected noise. The left ear control section 95a generates, based on the detection signal eL, a control signal for controlling a level of the detection signal eL such that the level is lowered. The left ear control section 95a outputs the generated control signal to the left ear speaker 93a. Similarly, the right ear microphone 94b detects a noise arriving in the right ear case 92b. The right ear microphone 94b outputs, as a detection signal eR to the right ear control section 95b, a noise signal based on the detected noise. The right ear control section 95b generates, based on the detection signal eR, a control signal for controlling a level of the detection signal eR such that the level is lowered. The right ear control section 95b outputs the generated control signal to the right ear speaker 93b. 
Next, configurations of the left ear control section 95a and the right ear control section 95b as well as processes performed by the left ear control section 95a and the right ear control section 95b will be described in detail with reference to FIG. 21. FIG. 21 shows, by blocks of signal processing, the configuration of the noise-canceling headphone of FIG. 20. It is assumed for FIG. 21 that components, which are denoted by the same reference numerals as those used for components in FIG. 20, have the same functions as those of the components in FIG. 20, and descriptions thereof will be omitted.
A block 921a in the left ear case 92a indicates an electroacoustic transfer function HL from an input of the left ear speaker 93a to an output of the left ear microphone 94a. A block 921b within the right ear case 92b indicates an electroacoustic transfer function HR from an input of the right ear speaker 93b to an output of the right ear microphone 94b. An adder 922a adds an output signal of the block 921a to a noise signal NL indicating the noise arriving in the left ear case 92a. A signal outputted from the adder 922a is the aforementioned detection signal eL. An adder 922b adds an output signal of the block 921b to a noise signal NR indicating the noise arriving in the right ear case 92b. A signal outputted from the adder 922b is the aforementioned detection signal eR.
First, a process performed for the left ear of the user 90 will be described. The left ear control section 95a comprises a feedback control filter 951a and a phase inverter 952a. For the feedback control filter 951a, a filter coefficient indicating a transfer function CL is set. The detection signal eL outputted from the adder 922a is inputted to the feedback control filter 951a. The phase inverter 952a inverts a phase of an output signal of the feedback control filter 951a. An output signal from the phase inverter 952a is inputted to the block 921a. Here, a transfer function from the noise signal NL to the detection signal eL is represented by an equation (1).
      (          equation      ⁢                          ⁢      1        )                                                          e              L                                      N              L                                =                      1                          1              +                                                C                  L                                ⁢                                  H                  L                                                                                          (          1          )                    
Here, the transfer function CL of the feedback control filter 951a is set, as shown in an equation (2), so as to have an inverse characteristic to that of the electroacoustic transfer function HL at the left ear. Note that, a indicates a filter gain of a fixed frequency.
      (          equation      ⁢                          ⁢      2        )                                            C            L                    =                      α                          H              L                                                            (          2          )                    
When the noise arrives in the left ear case 92a, the left ear microphone 94a outputs, as is clear from the equation (1), NL/(1+CL×HL) as the detection signal eL. The detection signal eL is inputted to the feedback control filter 951a. At this point, the control signal generated at the feedback control filter 951a is CL×NL/(1+CL+HL). Since the transfer function CL is set as shown in the equation (2), the control signal is NL/(HL×(1+1/α)). The control signal is inputted to the block 921a after a phase of the control signal is inverted at the phase inverter 952a. Accordingly, a cancellation sound, which is −HL×NL/(HL×(1+1/α))=−NL/(1+1/α), is radiated from the left ear speaker 93a to the vicinity of the left ear. As a result, the greater the filter gain α, the nearer to −NL the cancellation sound becomes, whereby the noise arriving near the left ear is canceled.
Next, a process performed for the right ear of the user 90 will be described. The right ear control section 95b comprises a feedback control filter 951b and a phase inverter 952b. For the feedback control filter 951b, a filter coefficient indicating a transfer function CR is set. The detection signal eR outputted from the adder 922b is inputted to the feedback control filter 951b. The phase inverter 952b inverts a phase of an output signal of the feedback control filter 951b. An output signal from the phase inverter 952b is inputted to the block 921b. Note that, the process performed for the right ear is different from the above-described process performed for the left ear only in that the transfer function CR of the right ear control section 95b has an inverse characteristic to that of the electroacoustic transfer function HR at the right ear. Other than this, the process performed for the right ear is the same as that of the process performed for the left, and therefore a description thereof will be omitted.
There is a known conventional technique in which the noise reduction function illustrated in FIG. 21 and an audio signal outputting function are combined. FIG. 22 shows a configuration in which the noise reduction function and the audio signal outputting function are combined. It is assumed for FIG. 22 that components, which are denoted by the same reference numerals as those used for components in FIG. 20, have the same functions as those of the components in FIG. 20, and descriptions thereof will be omitted.
A configuration shown in FIG. 22 is a result of adding, to the configuration shown in FIG. 20, an audio signal output section 97, a left ear audio signal canceling section 98a, a right ear audio signal canceling section 98b, subtractors 99a and 99b, and adders 100a and 100b. The audio signal output section 97 outputs audio signals such as music. As shown in FIG. 22, the audio signal output section 97 outputs an audio signal AL to the left ear and an audio signal AR to the right ear. The left ear audio signal canceling section 98a generates, based on a filter coefficient indicating a transfer function simulating the electroacoustic transfer function HL, a cancellation signal for canceling the audio signal AL. The subtractor 99a subtracts, from the detection signal eL, the cancellation signal for canceling the audio signal AL. An output signal from the subtractor 99a is inputted to the left ear control section 95a. A control signal outputted from the left ear control section 95a is added to the audio signal AL by the adder 100a. An output signal from the adder 100a is inputted to the left ear speaker 93a. The left ear speaker 93a outputs a sound based on the control signal and the audio signal AL.
Here, the detection signal eL from the left ear microphone 94a contains the audio signal AL. However, the subtractor 99a subtracts, from the detection signal eL, the cancellation signal for canceling the audio signal AL. As a result, the audio signal AL is not inputted to the left ear control section 95a, and the same process as that described in FIG. 21 is performed at the left ear control section 95a. 
The right ear audio signal canceling section 98b generates, based on a filter coefficient indicating a transfer function simulating the electroacoustic transfer function HR, a cancellation signal for canceling the audio signal AR. The subtractor 99b subtracts, from the detection signal eR, the cancellation signal for canceling the audio signal AR. An output signal from the subtractor 99b is inputted to the right ear control section 95b. A control signal outputted from the right ear control section 95b is added to the audio signal AR by the adder 100b. An output signal from the adder 100b is inputted to the right ear speaker 93b. The right ear speaker 93b outputs a sound based on the control signal and the audio signal AR. Other than the above, the process for the right ear is the same as the above-described process for the left ear, and therefore a description thereof will be omitted. As described above, the configuration shown in FIG. 22 allows noise reduction and stereo audio signal reproduction to be performed concurrently.
Usually, in a radio frequency band, a phase lag occurs in each of the electroacoustic transfer functions HL and HR. For this reason, there is a problem that even if, e.g., the transfer function CL is set to have an inverse characteristic to that of the electroacoustic transfer function HL, the transfer function CL does not have the inverse characteristic to that of the electroacoustic transfer function HL in the radio frequency band, whereby noise reduction effect deteriorates. For this problem, there is a conventionally suggested configuration as shown in FIG. 23 for widening a frequency band in which a noise reduction effect is obtained. FIG. 23 shows a configuration of a noise-canceling headphone capable of widening a frequency band in which a noise reduction effect is obtained. The configuration shown in FIG. 23 is a result of adding, to the configuration shown in FIG. 20, a left ear high frequency control section 101a, a right ear high frequency control section 101b and adders 102a and 102b. 
As shown in FIG. 23, the left ear control section 95a generates, based on the detection signal eL, a control signal for controlling a level of the detection signal eL such that the level is lowered, the control signal having a frequency which is no higher than a predetermined frequency. In other words, the left ear control section 95a generates a cancellation signal for canceling a noise arriving in the left ear case 92a, the noise having the frequency which is no higher than the predetermined frequency. Here, the predetermined frequency is lower than a frequency at which a phase lag of the electroacoustic transfer function HL occurs. The left ear control section 95a outputs the generated control signal to the adder 102a. The left ear high frequency control section 101a generates, based on the detection signal eL, a control signal for controlling the level of the detection signal eL such that the level is lowered, the control signal having a frequency which is higher than the predetermined frequency. In other words, the left ear high frequency control section 101a generates a cancellation signal for canceling a noise arriving in the left ear case 92a, the noise having the frequency which is higher than the predetermined frequency. The left ear high frequency control section 101a outputs the generated control signal to the adder 102a. The adder 102a adds the control signal generated at the left ear control section 95a to the control signal generated at the left ear high frequency control section 101a. A signal resulting from the addition at the adder 102a is inputted to the left ear speaker 93a. The left ear speaker 93a outputs sounds based on the control signals generated at the left ear control section 95a and the left ear high frequency control section 101a. As a result, the sounds, which are based on the control signals, and the noises are canceled by each other near the left ear.
On the other hand, the right ear control section 95b generates, based on the detection signal eR, a control signal for controlling a level of the detection signal eR such that the level is lowered, the control signal having a frequency which is no higher than a predetermined frequency. In other words, the right ear control section 95b generates a cancellation signal for canceling a noise arriving in the right ear case 92b, the noise having the frequency which is no higher than the predetermined frequency. Here, the predetermined frequency is lower than a frequency at which a phase lag of the electroacoustic transfer function HR occurs. The right ear control section 95b outputs the generated control signal to the adder 102b. The right ear high frequency control section 101b generates, based on the detection signal eR, a control signal for controlling the level of the detection signal eR such that the level is lowered, the control signal having a frequency which is higher than the predetermined frequency. In other words, the right ear high frequency control section 101b generates a cancellation signal for canceling a noise arriving in the right ear case 92b, the noise having a frequency which is higher than the predetermined frequency. The right ear high frequency control section 101b outputs the generated control signal to the adder 102b. The adder 102b adds the control signal generated at the right ear control section 95b to the control signal generated at the right ear high frequency control section 101b. A signal resulting from the addition at the adder 102b is inputted to the right ear speaker 93b. The right ear speaker 93b outputs sounds based on the control signals generated at the right ear control section 95b and the right ear high frequency control section 101b. As a result, the sounds, which are based on the control signals, and the noises are canceled by each other near the right ear.
As described above, separately for a high frequency band higher than the predetermined frequency in which a phase lag of the electroacoustic transfer function occurs, controls are performed using the left ear high frequency control section 101a and the right ear high frequency control section 101b for each of which a filter coefficient is set based on the electroacoustic transfer function whose phase is lagged. This allows a frequency band, in which the noise reduction effect is obtained, to be widened.    [Patent Document 1] (PCT) International Publication WO94/17512